1. Field of the Invention
This invention relates to high density integrated circuit devices. More particularly, this invention relates to an external bus communication system for high density integrated circuit devices stacked to form ultra high density integrated circuit packages, or modules.
2. Discussion of Related Art
Designers of computers and other electronic systems constantly strive to use integrated circuit devices which are smaller and more complex. Because of this demand, there is a need to develop smaller, yet denser, memory packages, or modules, and other small, yet dense, modules containing dissimilar integrated circuit devices, such as microprocessors, memory devices, DMA devices, etc. Typically, these modules require external electrical connections to other modules within the same electronic device or external electronic circuits, and internal communication paths, or buses, for data communication between the discrete semiconductor elements within the module itself.
The problem then has arisen with regard to how to create electronic pathways, or buses, for stacked high density integrated circuit modules which are physically external to the high density module and provide an external communication path from each integrated circuit package within the module to external electronic devices, and a communication path for communication between individual integrated circuit packages within the module.
Various schemes have been developed to provide these buses. A rail bus system is described in application Ser. No. 08/059,401, pending, owned by the common assignee of the present invention. An alternative, rail-less bus system is exemplified in U.S. Pat. No. 4,884,237.
Rail-less bus systems are typically less costly to produce. A major drawback with known rail-less bus systems is that a maximum of two industry standard integrated circuit devices, or level-one packages, can be vertically stacked, with non-common signal external leads on either the upper or lower package jumpered together, and common signal leads on each package placed into contact with each other. This type of rail-less bus scheme described in U.S. Pat. No. 4,884,237 is additionally limited because it requires that standard packages with industry standard pin assignments be used, including a chip select pin and a no-connect pin.